Introduction 283
allows the measurement to extend far into the wings of the mode-locked spectrum,
greater than 50 dB down from the center peak. The repetition rate of the different
wavepackets does not vary as a function of frequency or wavelength over a total
span of 250 nm. It is the nonlinear phase shift because of the mode-locking
mechanism (in this case the Kerr modulation) that compensates for the GVD.
Such a result is expected, because the different wavepackets formed with any
group of modes should all travel at the same group delay, or they will not produce
a pulse that “stays together” after several round-trips. It is also consistent with the
Fourier transform of an infinite train of equally spaced pulses, which produces a
comb of equally